Magnetism AP Physics Chapter 20

Magnetism 20.1 Mangets and Magnetic Fields

Magnets and Magnetic Fields Objects that align with the magnetic field of the earth. The north pole is magnetic south pole Like poles repel and unlike poles attract Unlike charges, poles can not be isolate If a magnet is cut, new poles appear 20.1

Magnets and Magnetic Fields Magnetic field lines – look similar to electric field lines Point in the direction of a force on a North monopole Interaction is also similar to electric fields 20.1

Magnets and Magnetic Fields The earths magnetic field – Caused by the iron core Does not point to geographic North Provides protection against solar wind 20.1

Magnetism 20.2 Electric Currents Produce Magnetic Fields

Electric Currents Produce Magnetic Fields Hans Christien Oerseted – 1820 a compass needle deflects when a current is first produced in a wire The field is in a circle around the wire We can determine the direction of a magnetic field using the right hand rule 20.2

Electric Currents Produce Magnetic Fields Right hand rule #1 1. the thumb points in the direction of the current 2. the fingers curl in the direction of the magnetic field A magnetic field due to a loop 20.2

Magnetism 20.3 Force on an Electric Current in a Magnetic Field

Force on an Electric Current in a Magnetic Field Magnets exert force on a current-carrying wire (Newton’s Third Law) The force is 1. perpendicular to the direction of current 2. perpendicular to the direction of the magnetic field 20.3

Force on an Electric Current in a Magnetic Field Right Hand rule #2 1. fingers, direction of magnetic field 2. thumb, direction of current 3. palm, direction of force The force is defined as B – magnetic field measure in tesla (T) – vector l – length of wire in the field 20.3

Force on an Electric Current in a Magnetic Field A tesla is also called a weber per meter squared The units in cgs is called a Gauss 20.3

Magnetism 20.4 Force on Electric Charge Moving in a Magnetic Field

Force on Electric Charge Moving in a Mag Field The equation can be determine by an analysis of the current I is the amount of charge that passes a point in a given time l can be defined as Putting those in the force equation 20.4

Force on Electric Charge Moving in a Mag Field The force is maximum when the charged particle moves perpendicular to the field Zero when it moves parallel to the field The angle is the angle between the velocity and the field The right hand rule will give you the correct direction for the force on a positive charge 20.4

Force on Electric Charge Moving in a Mag Field If we establish a uniform magnetic field and the charge moves perpendicular to the field In our example -Field (fingers) is out of the slide -Velocity (thumb) is in the direction of the blue arrow -Force (palm) – points toward middle (assume positive charge) The force is centripetal 20.4

Force on Electric Charge Moving in a Mag Field A very common type of problem The electromagnetic force is the centripetal force Charged particles from the sun, concentrate at the poles because of this – called the Aurora Borealis 20.4

Magnetism 20.5 Magnetic Field Due to a Long Straight Wire

Magnetic Field Due to a Long Straight Wire As discussed earlier, a current in a wire produces a magnetic field This can be demonstrated by moving a compass around a current carrying wire. 20.5

Magnetic Field Due to a Long Straight Wire The field around the wire is directly proportional to the current and inversely to the distance from the wire  0 is called the permeability of free space 20.5

Magnetism 20.6 Force Between Two Parallel Wires

Force Between Two Parallel Wires Assuming that two wires are parallel to each other 20.6

Magnetism 20.9 Torque on a Current Loop; Magnetic Moment

Torque on a Current Loop Electric Motor 20.9

Magnetism Applications

Applications Galvanometer 1.Current flows through the wire 2.Produces a magnetic field 3.The magnetic field is attracted or repelled by the permanent magnet Found in analog voltmeters, ammeters, and ohmeters 20.10

Applications Electric Motor 1.Current flows through the wire 2.Produces a magnetic field 3.The magnetic field is attracted or repelled by the permanent magnet 4. Brushes allow the current to switch 20.10

Applications Loudspeaker 1.Current from the amp flows through the wire 2.Produces a magnetic field 3.This pushes the coil away, causing the cone to move outward 4. Signal turns off, or switches, the cone moves back 20.10